To map, purify and study the repair deficiencies after ionizing irradiation of a new class of mutants of E. coli that we have isolated that are largely UV resistant but are ionizing radiation sensitive. The strains currently being purified are called irrB and irrC, and map near leu, and dnaA, respectively. Strains 41, 205, 211 and 232 map near but do not cotransduce with his. Additional strains are being developed using transposon insertion mutagenesis. The irrA strain appears to modify rich growth medium-induced resistance to radiation damage, a process controlled by the recA, recB, and lexA genes.